LT5522

13

5522fa

Higher linearity and lower LO-IF leakage can be realized by

using the simple, three element lowpass matching net-

work shown in Figure 10. Matching elements C4, L1 and

L2 form a 400

Ω to 200Ω lowpass matching network

which is tuned to the desired IF frequency. The 4:1

transformer then transforms the 200

Ω differential output

to 50

Ω single-ended. The value of C4 is reduced by 1pF to

account for the equivalent internal capacitance.

For optimum linearity, C4 must be located close to the IF

pins. Excessive trace length or inductance between the IF

pins and C4 will increase the amplitude of the image output

and reduce voltage swing headroom for the desired IF

frequency. High Q wire-wound chip inductors (L1 and L2)

improve the mixer’s conversion gain by a few tenths of a

dB, but have little effect on linearity.

This matching network is most suitable for IF frequencies

of 40MHz or above. Below 40MHz, the value of the series

inductors (L1 and L2) is high, and could cause stability

problems, depending on the inductor value and parasitics.

Therefore, the 8:1 transformer technique is recommended

for low IF frequencies.

Suggested matching network values for several IF fre-

quencies are listed in Table 3. Measured output return

losses for the 140MHz match and the wideband CATV

match are plotted in Figure 13.

Table 3. IF Matching Element Values (See Figure 10)

IF FREQUENCY

L1, L2

C4

(MHz)

(nH)

(pF)

IF TRANSFORMER

2-140

Short

—

TC8-1 (8:1)

70

220

4.7

ETC4-1-2 (4:1)

140

82

1.5

240

56

0.5

380

39

—

50-1000 (CATV)

18

—

MABAES0054 (4:1)

For fully differential IF architectures, the IF transformer

can be eliminated. As shown in Figure 14, supply voltage

to the mixer’s IF pins is applied through matching induc-

tors in a bandpass IF matching network. The values of L1,

L2 and C4 are calculated to resonate at the desired IF

frequency with a quality factor that satisfies the required IF

bandwidth. The L and C values are then adjusted to

account for the mixer’s internal 1pF capacitance and the

SAW filter’s input capacitance. In this case, the differential

IF output impedance is 400

Ω, since the bandpass network

does not transform the impedance.

For low cost applications, it is possible to replace the IF

transformer with a lumped-element network which pro-

duces a single-ended 50

Ω output. One approach is shown

in Figure 15, where L1, L2, C4 and C6 form a narrowband

bridge balun. The L and C values are calculated to realize

a 180 degree phase shift at the desired IF frequency using

the equations listed below. Inductor L4 is calculated to

cancel the internal 1pF capacitance. L3 also supplies bias

are adequate for L1 and L2. A high Q wire-wound chip

APPLICATIO S I FOR ATIO

Figure 13. Typical IF Output Return Losses

for Various Matching Techniques

Figure 14. Bandpass IF Matching for Differential IF Architectures

IF FREQUENCY (Hz)

0

–5

–10

5522 F13

–25

–15

–20

1E9

1E7

1E8

240MHz MATCH

LUMPED ELEMENT

BRIDGE BALUN

LOW FREQ MATCH

(NO IF MATCHING)

8:1 BALUN

140MHz MATCH

(82nH/1.5pF)

4:1 BALUN

50MHz TO 1000MHz

(18nH/0pF)

4:1 CATV BALUN

SAW

FILTER

C3

C4

L2

5522 F14

L1

IF

AMP